I just upgraded my Samsung S9 from v0.23 to v1.0 of /E/
I ended up in /e/ recovery app.
Tried to get into TWRP to restore, but kept on ending up in /e/ recovery app.
So selected ‘Factory reset’. Now, boot was successful and I can start re-installing the phone.
But still can not boot into TWRP: bixy+power+volume up makes me end up in this new /e/ recovery app.
Is this normal? Is TWRP out? Or is there another way to boot into TWRP?
Yes, TWRP is gone. In these modern times the recovery may be part of the boot image or some such. Either way, you will lose it when updating and flashing ROMs. Yet another reason why I don’t like the state of recent Android.
Anyway, you’ll have to reinstall TWRP.
Thank you! Your answer helps me understand this world of ROMs again a little bit better. 
Installing TWRP on /E/-SamsungS9 appeared to be a challenge (for me).
I’m on Windows 11. It started of easy with this clear manual: Install TWRP
But fastboot flash command ended in < waiting for any device >.
Reason: Samsung does not have a fastboot mode (see ://stackoverflow.com/a/70178103 and reaction below of @aibd)
Alternative: Use Odin as explained here. But the focus of this tutorial is on non-rooted Samsung phones.
So follow it carefully, but realize that when /E/ is already installed, there are some subtle differences:
VolumeDown+Power button will power off the phone (not restart)Bixby+Power+VolumeUp and release after logo appears.I hope this helps someone.
PS. Sorry for the weird inline links. As a new user I’m only allowed to post max two links.
Samsung does not have fastboot, we find Download mode aka Odin mode! The tool to use is heimdall.
( … or Odin for Windows)
Some intro level information here Know your hardware - Samsung
@Manoj, I thought I had a memory that this page Install TWRP was adjusted to say tell users Unsuitable for Samsung at one point in the past. Please can that be ameliorated.
The guide for the s9 was modified recently as per the instructions from the developers. Will have it reviewed and updated.
/e/ users wishing to preserve TWRP and not have it overwritten at update should in the first place ensure that
Settings > System > Developer options > Update recovery is not enabled.
The location of this switch has changed in /e/ v1.0 please see Post #13, thanks @marcdw.
I also wanted to boot into TWRP, but somehow the phone (fp3) is locked while in fastboot, wanted to adb shell input text xxxx but lsusb is printing different phones (redmi and pixel), so it’s somehow not connected or recognized anymore, inputs do not work. What to do? 
I think we cannot expect to run adb commands from fastboot mode.
I expect adb to work on a phone either regularly powered on or in Recovery.
I have no experience of /e/ recovery, but I would expect adb to work there.
I’m pretty sure that was disabled in my case, as that is the default value and I never touched it. But ‘pretty sure’ is not 100% of course.
In doing a v1.0-r fresh start wizard for the first time last night I saw a small “invitation dialog” (which might easily be missed, I took no screenshot) which seemed to invite the user to have Recovery overwritten fresh at update!
I was looking for that update recovery toggle in developer options and couldn’t find it. I then found it in the System Updates Preferences (three-dot menu). /e/OS 1.0.
I wish Carbon ROM on my OP8T had that option. I tire of having to reinstall OrangeFox recovery after each update.
New name for me. I’ll look into that one.
=== edit ===
pity, OrangeFox supports Galaxy S9 starqltechn but not S9 starlte.
FYI, when I installed from scratch using TWRP, the “new” /e/ R on my A5 2017, indeed, TWRP disappeared.
Why? Because, I think that the reason is that I enabled the option below in the first start screens:
So to keep TWRP, I would say that you have to:
it’s what I observed and tested this evening. If that can help
