This package [1] on f-droid is built directly of the source on github [2]. Even if the source is old so is the package. F-droid compiles this package not telegram.
I wrote the peertube client Throium which is also available on f-droid [3]. When I Tag a new release on github, f-droid will build the package automatically and publish it the next day. I do not build the packages for f-droid.
I gave up tracking the delay a while ago but I think the macOS and iOS packages were lagging by six months at one point. Heck, even now the last commit with actual code [1] is over two months old yet the macOS app updated two days ago for me.
Uncommented code is dumped into the GitHub repo every couple of months after enough people complain. That's not what I would call open source.
I wrote the peertube client Throium which is also available on f-droid [3]. When I Tag a new release on github, f-droid will build the package automatically and publish it the next day. I do not build the packages for f-droid.
[1] https://f-droid.org/en/packages/org.telegram.messenger/
[2] https://github.com/Telegram-FOSS-Team/Telegram-FOSS
[3] https://f-droid.org/en/packages/net.schueller.peertube/