Edit: After every update of the app, the DTTG share button keeps disappearing from the share sheet and restart is needed for it to appear again. (DTTG button usually disappear after once or twice I use it to share content to DTTG).
After the restart, it usually works fine.
I use IpadOS 14.3 on my Ipad Pro 2018.
This is true for me as well on my iPad (6th generation). However on my iPhone 7 the sharesheet works.
Turning the device off an on was enough to DTTG to appear in sharesheet. This was not only limited to DTTG, but a more general issue with my IpadOS system because more apps weren’t present in the sharesheet (some of the recently downloaded or updated). After turning off/on they appear in sharesheet.
That worked also for me 
Oh good, it’s not just me. I haven’t had a chance to restart yet but that didn’t fix another app that recently vanished on me (DuckDuckGo).
it probably Apple fault, but within 14.4 DTTG3 in sharesheet is still sometimes disseappering (sometimes it come back back for a moment on the same page, but then again in not available ), restart probably will help
This is still happening, after every update, the DTTG share button keeps disappearing and restart is needed.
This is a known problem of iOS. Happens with other apps, too.
Maybe the devs should add info about this to release notes as known bugs, since now the new updates are much frequent
I can’t speak for the devs. But given that it is an iOS problem, maybe Apple should talk about it in their release notes. Instead of every app’s developer doing it.
Yeah, so annoying. Even iOS 14.5 beta 2 doesn’t fix it.
Apple has a general problem with admitting to their bugs.
I saw today another topic about disappearing sharing sheet, it’s not the defeloper fault but having info about that in release notes maybe would save some users who don’t know about the issue from some problems.
This is only a proposition, developers will do what they want 
It’s certainly an iOS issue. Pocket has updating pretty frequently, and it’s an issue I notice much more with Pocket. I will sometimes refrain from updating it if I don’t have time to reboot my device right after the update. It’s not happening to 100% of people with 100% of updates, but it happens a lot.
I believe this is not only happening after app updates, but rather much more frequently and sporadically. I feel like I am running into the issue nearly every day and it’s driving me a little nuts. It’s also kind of random in which apps it happens, e.g. it will disappear in Safari but still be available in Reeder’s article view, then some other time it’ll be gone in Reeder’s built-in Safari but still available it real Safari.
This was also happening with Instapaper for a while but I haven’t noticed it as often recently. With DTTG 3 it’s a constant show-stopper to my workflow.
Funny enough it just popped up again after writing this. I neither got any updates nor rebooted the device.
Aaaand I switched over to Things to grab a URL I wanted to use the web clipper with and it’s gone again in Safari.
Aaand it’s back for a few taps. And it’s gone again. I don’t get it.
Edit: I got a screen recording of it. Don’t want to share it here as it has my contacts in it but happy to send it to the developers for further inspection.
You’re describing abnormal behavior. Are you running the iOS beta?
Nope I never run iOS betas, no time for beta bugs as I deal with enough of those in my day to day as a web developer. 
I just DM’d you the screen captures. iOS 14.4, latest update of DTTG3, no betas.