Hey people!
I played Undertale about a year after release, but accidentally did not do a pacifist run, resulting in me not being able to get the "true" ending without fully playing the game from start again. I left it there, pretty happy with my experience.
A month ago I decided to give it another go, mainly because of an interest in the earthbound series and also because I might try out Deltarune and wanted to be up to speed.
This game is just so... nice. The subjects, the insecurities that are mentioned, the hopefulness, the acceptance of everyone that just permeates throughout the whole work, it's astounding. The characters are all so imperfect and charming in their own way. I've been going through a tough period myself and a lot of the messages about acceptance of who you are and connection with people you love just really hit home.
I know this game is already highly regarded and I'm saying nothing new here, but I just wanted to share what it did for me and that it really, really holds up.
Once you finished the final boss, you can re-load your last save and there are extra bits you need to do in order to get the full pacifist run and second final boss.
The game won't start the events that take you there without first completing the neutral run final boss, even if you did a full pacifist run on your first play through.
So, I have only played through the entire game once and it was a Pacifist run. I did the full extended run, reloading my last save and everything.
I started a Neutral run later, but I haven't even completed it yet.
Edit: I just googled it. You have to view the neutral ending once, which you get at the end of the Pacifist run before reloading. You do not need to do a full Neutral run.