Big Sur seems to revert to SMB1?

I will try that and report back. Thank you.

According to another post with no replies except the OP reporting. He originally said that Big Sur 11.3 and higher did not include SMB2 or higher, and then later speculated that on some MBP's 11.3 forced a revert to SMB1, since he was able to restore SMB2. The first command line from him was identical to the one in your link. I only figured this out because I added a logging program from Synology, and captured an error message "SMB1 not permitted". The MBP was using SMB1 which I have prevented from the NAS from accepting.

Just in case someone else has this issue...

Upgrade to latest Big Sur, and suddenly, my MBP cannot find the shared folder for Time Machine on a Synology NAS. Logs said "SMB1 not permitted", which I take to mean the MBP was using SMB1 to connect and I have SMB1 disabled in the shared folder for both security and efficiency reasons. So the MBP would attempt to connect to the shared folder and fail. This only happened on a MBP. 2 iMacs running the same set-up and same net/NAS worked fine.

I found one other person with this issue. His suggestion was to use terminal to create a prefs file and then enable SMB2 (as in my OP), which made me a little nervous. So I tried this and it worked.

Go to NAS software and enable AFP, but leave SMB at min SMB2 and max SMB3. restart computer. Open Time Machine prefs, delete shared folder as destination, then choose same folder from menu. Click back up now. MBP will see the folder and attempt a back up, which will eventually fail (because of AFP). Go back to NAS software and disable AFP, close, restart MBP, and you should be good. I don't know why this worked, but after the NAS and MBP communicated, things were back to normal, with SMB1 still disabled.

Exactly. I set the SMB on the NAS to accept either SMB2 or SMB 3. Mac OS is supposed to not use SMB1 any longer, but there was a bug in 11.3 according to another post on this issue. For whatever reason, the bug seemed to only affect MBP of a certain vintage. The solution was to use terminal and create a pref file for SMB manually, and then change it to SMB2. I tried another workaround, described above, which worked. I know what Apple is supposed to use, but somehow Apple disabled all but SMB1 in 11.3 and later updates in some selected computers. (I'm on 11.6 and had the issue). I don't know which version of SMB - 2 or 3 - my MBP is using, since I don't know how to find that value.

The workaround on that other post from May was:

Enter the folowing commands on the terminal window.

echo "[default]" | sudo tee -a /etc/nsmb.conf

echo "protocol_vers_map=2" | sudo tee -a /etc/nsmb.conf

SMB Version 2 will be enabled.

I can't vouch for this but the post indicated it would work.

I only repeated all of this in case someone else went looking for the same issue. It was very aggravating trying to diagnose until I enabled a Synology logger. Simple, I know, but I missed it for a time.