I don't see anything wrong in git branch either:

% git clone -b release-2.3.16 https://github.com/dovecot/core.git
% git grep fts_user_autoindex_exclude
%

% git show
commit 7e2e900c1a420006371e8a8cf9b4ddb36e69a986
Author: Timo Sirainen <timo.sirainen@open-xchange.com>
Date:   Thu Aug 5 18:25:31 2021 +0300

    NEWS: Add 100% master process CPU item

I think somehow you built it from master instead of release-2.3.16.

On 6. Aug 2021, at 16.28, Joan Moreau <jom@grosjo.net> wrote:

git clone -b release-2.3.16


 


On 2021-08-06 15:07, Timo Sirainen wrote:

On 6. Aug 2021, at 15.08, Joan Moreau <jom@grosjo.net> wrote:

Below

(gdb) bt full
#0 fts_user_autoindex_exclude (box=<optimized out>, box@entry=0x55e0bc7e0fe8) at fts-user.c:347

 
There is no such function in 2.3.16 release. That's only in the current git master. What did you install and from where?