Done a quick search, couldn't find one for 1.15 right away, but there might be one. Also, really not needed as a plugin anyways, since servers being in online mode and with a normal level of protection would be save anyways. If its for a cracked server, c'mon, this game has been out for what, 10 years? more? Are cracked servers still a big thing ? Oof...
You can try using AuthMeReloaded dev builds. Link: https://ci.codemc.io/job/AuthMe/job/AuthMeReloaded/ These may have some bugs, first try it on a test server.
Are you running a cracked server? Authme is only needed for cracked server. If you are running a cracked server then don't do it. It is strongly against the EULA.
The 'option' of offline mode is in the base server file for people who want to run a server on their own machine or system but not have it open to other people or purely for testing. The fact that people use this setting, as a bypass for allowing cracked clients to join, and then add plugins to have their own login etc. People then publicly advertise these servers and at this point, its in breach of the EULA and general Terms of Use.
I only found these https://www.spigotmc.org/resources/securelogin-secure-authentication-service.69051/ https://www.spigotmc.org/resources/...gin-the-premium-players-■-1-8-1-15-1-■.27766/
Technically, it's not illegal. If it wasn't for a option for servers to be in offline mode, than bungeecord or anything similar to that, simply wouldn't exist as certain sections on bungeecord need to be in offline mode, not online mode. I'm not supporting anyone running a cracked server, if you seen my previous posts, I am quite against it. I mean..... how hard is it to get $25 to get a minecraft account? However, there is nothing wrong with just helping someone on forums. There are a lot of cracked servers out there, the difference is you don't need to have a premium minecraft account to join. However, you shouldn't judge and just assume someone is "doing illegal stuff" for all we know, the OP could just be asking for a /login plugin for staff or something. I ran servers that have 2FA which technically counts as a login plugin to authenticate yourself. FYI, /login isn't just for cracked servers, it's for premium as well. That extra layer of security can be vital.