I have no idea what happened with Tabula but he seemed quite downtrodden so I hope he's okay. Current theories:
1. He's Myst's older brother and Myst told his parents that Tabula's letting strangers bully him online, earning lots of flack from Mrs. Rasa so he had to leave.
2. He's head of the Kookaburra Intelligence Agencies and just finished up his data scraping project on the radicalization of neurodivergents so has no need for the forum anymore.
3. Renty-chan (rentier) did freaky Russian blackmail on Tabula to get him to depart because he's plotting a secret takeover.
4. Tabula is secretly a woman and got 'the ick' from all of the incels coming to his/(her?) forum.
5. Tabula got hounded by the feds because too many sensitive individuals are coming here and was so scared he left.
6. Tabula doesn't want to be associated with this forum because he started university and doesn't want to be associated with smelly incels.