AOL IM disappeared after installing High Sierra
Looks to be that way. The only other Messages account type is Jabber.
Yes, AOL has already discontinued support...
https://arstechnica.com/business/2017/02/aol-will-cut-off-third-party-app-access-to-aim/
Also, I noticed, that even AOL in general is disabled. I would say it's safe to say if you use AOL based email, you cannot even get your own email in High Sierra. I was testing it to see if that works, and it tried to connect but never does, and I know this AOL Mail account is active and worked in Sierra.
I don'ty buy this. I have 2 other macs running AOL IM on Sierra ...and still do today. Your link says it would no longer work after March 28...but still runs today. Even if say this is something they will do, they would not kill AOL Mail...which also does not work on High Sierra.
And nothing of value was lost.
Get a new copy here, a little different but works.
Apple’s removal of AIM support in High Sierra was purposefully timed, it seems.
AOL is shutting down the AIM service entirely on December 15, 2017.