Mapping using <Esc> writes "<Esc>" in insert mode rather than switching to normal mode

Question

I have the following line in my .vimrc:

inoremap kj <Esc>

It had been working fine, ie it returned me to normal mode. But now it inserts the string "< Esc>" (I use the initial space here just so my post displays properly) into the file at the cursor and I remain in insert mode.

My full .vimrc is as follows

syntax on
set smartindent
set shiftwidth=4
set expandtab
set tabstop=4
inoremap kj <Esc>
nnoremap zz :update<cr>
nnoremap qq :wq<cr>
nnoremap mm :vsplit<cr><C-w><C-w>
" Ctrl-j/k deletes blank line below/above, and Alt-j/k inserts."
nnoremap <silent><C-j> m`:silent +g/\m^\s*$/d<CR>``:noh<CR>
nnoremap <silent><C-k> m`:silent -g/\m^\s*$/d<CR>``:noh<CR>
nnoremap <silent><A-j> :set paste<CR>m`o<Esc>``:set nopaste<CR>
nnoremap <silent><A-k> :set paste<CR>m`O<Esc>``:set nopaste<CR>
nnoremap <Enter> i<Enter><Esc>
nnoremap <S-Enter> o<Esc>
nnoremap <S-tab> i<tab><Esc>l
nnoremap <C-x> ^i#<Esc>
nnoremap <C-d> yy^i#<Esc>p

Why has this started happening and how can I fix it?

EDIT: output of the following commands

:set compatible?
nocompatible

:set cpoptions?
cpoptions=aAbBcCdDeEfFgHiIjJkKlLmMnoOpPqrRsStuvwWxXyZ$!%*-+<>;

EDIT2: I'm running Arch Linux, 4.19.56-1-lts, and the latest version of Vim. Output of "vim --version":

VIM - Vi IMproved 8.1 (2018 May 18, compiled Jul  4 2019 23:02:48)
Included patches: 1-1635
Compiled by Arch Linux
Huge version without GUI.  Features included (+) or not (-):
+acl               -farsi             -mouse_sysmouse    -tag_any_white
+arabic            +file_in_path      +mouse_urxvt       +tcl/dyn
+autocmd           +find_in_path      +mouse_xterm       +termguicolors
+autochdir         +float             +multi_byte        +terminal
-autoservername    +folding           +multi_lang        +terminfo
-balloon_eval      -footer            -mzscheme          +termresponse
+balloon_eval_term +fork()            +netbeans_intg     +textobjects
-browse            +gettext           +num64             +textprop
++builtin_terms    -hangul_input      +packages          +timers
+byte_offset       +iconv             +path_extra        +title
+channel           +insert_expand     +perl/dyn          -toolbar
+cindent           +job               +persistent_undo   +user_commands
-clientserver      +jumplist          +postscript        +vartabs
-clipboard         +keymap            +printer           +vertsplit
+cmdline_compl     +lambda            +profile           +virtualedit
+cmdline_hist      +langmap           +python/dyn        +visual
+cmdline_info      +libcall           +python3/dyn       +visualextra
+comments          +linebreak         +quickfix          +viminfo
+conceal           +lispindent        +reltime           +vreplace
+cryptv            +listcmds          +rightleft         +wildignore
+cscope            +localmap          +ruby/dyn          +wildmenu
+cursorbind        +lua/dyn           +scrollbind        +windows
+cursorshape       +menu              +signs             +writebackup
+dialog_con        +mksession         +smartindent       -X11
+diff              +modify_fname      -sound             -xfontset
+digraphs          +mouse             +spell             -xim
-dnd               -mouseshape        +startuptime       -xpm
-ebcdic            +mouse_dec         +statusline        -xsmp
+emacs_tags        +mouse_gpm         -sun_workshop      -xterm_clipboard
+eval              -mouse_jsbterm     +syntax            -xterm_save
+ex_extra          +mouse_netterm     +tag_binary        
+extra_search      +mouse_sgr         -tag_old_static    
   system vimrc file: "/etc/vimrc"
     user vimrc file: "$HOME/.vimrc"
 2nd user vimrc file: "~/.vim/vimrc"
      user exrc file: "$HOME/.exrc"
       defaults file: "$VIMRUNTIME/defaults.vim"
  fall-back for $VIM: "/usr/share/vim"
Compilation: gcc -c -I. -Iproto -DHAVE_CONFIG_H   -D_FORTIFY_SOURCE=2  -march=x86-64 -mtune=generic -O2 -pipe -fno-plt -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1       
Linking: gcc   -L. -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -fstack-protector-strong -rdynamic -Wl,-export-dynamic -Wl,-E -Wl,-rpath,/usr/lib/perl5/5.30/core_perl/CORE  -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -L/usr/local/lib -Wl,--as-needed -o vim        -lm -ltinfo -lelf -lnsl    -lacl -lattr -lgpm -ldl   -Wl,-E -Wl,-rpath,/usr/lib/perl5/5.30/core_perl/CORE -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now -fstack-protector-strong -L/usr/local/lib  -L/usr/lib/perl5/5.30/core_perl/CORE -lperl -lpthread -ldl -lm -lcrypt -lutil -lc   -L/usr/lib -ltclstub8.6 -ldl -lz -lpthread -lm 

Answer 1

TL;DR: You're in compatible mode, add this to the top of your .vimrc:

set nocompatible

Key specifiers using the <...> syntax are incompatible with the traditional vi and are specific to Vim.

This particular feature can be enabled by removing < from the ['cpoptions'] setting, which controls compatibility options. The < option is documented as:

Disable the recognition of special key codes in <> form in mappings, abbreviations, and the "to" part of menu commands. For example, the command :map X <Tab> results in X being mapped to:

• < included: <Tab> (5 characters)
• < excluded: ^I (^I is a real )

When in compatible mode, all options in 'cpoptions' are set, while only a handful (and not including <) are set in nocompatible mode.

So ensuring you're in nocompatible mode is the first step in making sure these mappings are going to work for you.