It's not possible to detect concealment from matchadd

[resolritter]

Is your feature request about something that is currently impossible or hard to do? Please describe the problem.

If a [row,col] position is concealed through matchadd("Conceal", ...) then there's no way to detect its concealment, unlike positions concealed through syn match [pattern] which can be detected through synconcealed.

Demonstration

set conceallevel=2 concealcursor=nvic
call feedkeys("i \<esc>", "n")
call matchadd("Conceal", '^\s*', 10, -1, { "conceal": "" })
fun! OnVimEnter(...)
  echo synconcealed(line('.'), col('.'))
endfun
autocmd VimEnter * call timer_start(0, function("OnVimEnter"))

Using the above script, [0, '', 0] will be printed from synconcealed although the character is, in fact, concealed.

The same approach, but replacing matchadd("Conceal",...) for syn match [pattern]:

set conceallevel=2 concealcursor=nvic
call feedkeys("i \<esc>", "n")
syntax match Concealment /^\s*/ conceal
fun! OnVimEnter(...)
  echo synconcealed(line('.'), col('.'))
endfun
autocmd VimEnter * call timer_start(0, function("OnVimEnter"))

Using the above script, [1, '', 1] will be printed from synconcealed and that can be used to detect the concealment.

Describe the solution you'd like

Extend synconcealed to also work for characters concealed through matchadd or create a new matchconcealed API for detecting them.