Thank you for your review! Changing the prefix is possible in the earlier versions (v2.6.2 and below) and in the premium version. Alternatively, you can make the change in the source code from GitHub and recompile.
There is also a giveaway of the premium version due to 30k downloads in the link below if you're interested.
https://www.spigotmc.org/threads/giveaway-3x-bettersleeping-premium.444072/
For an official statement regarding the removal of the prefix option, consider my explanation below:
I understand your need for a custom prefix, so I'm sorry it is no longer available in the free version.
That option was added in an early version of BetterSleeping. But I regretted that feature almost instantly as it did not spread the word of this plugin. So when rewriting the plugin from scratch, I decided to remove it from future builds.
While finishing up the update, I asked myself: Why not create a premium plugin to support the time I put into this? When drafting the concept, I thought of this: These users would be paying, they should have access to full customisability so I re-added the prefix option for them along with GUIs.
My philosophy is that with the split of free (no custom prefix) and premium (custom prefix), everyone would contribute to BetterSleeping's future. The free users would spread the word, and the premium users provide a one time monetary aid that one day could lead to a BetterSleeping website, server (or something else cool).
I really hope you do understand. I do provide free lifetime upgrades to premium for people that are helpful on the Discord server and/or do beta testing, but that is at my discretion and may take a while.
Another free option you have, is remaining on v2.6.2. I don't see a direct reason for that version to break in Minecraft 1.16 so that should be a safe bet for now (and I still provide support for v2!). In case it would break, I would see what I can do to fix it.
Buying the premium version is an option of course but I will never force that on users. So I really do hope the free alternatives can satisfy you.