Forgive me if my definition of crossover if off, but haven't there always been crossovers?
1) TNG premiere. Bones.
2) DS9 premiere. Picard and O'Brien.
3) VOY premiere. Quark and Morn.
4) ENT premiere. Zefram Cochrane.
5) DSC premier. Sarek (diff actor).
6) PIC premiere. Duh.
7) LD premiere. NOMAD.
8) PRO premiere. Janeway.
9) SNW premiere. Uhura. Diff actor.
Every premiere has had one. Shatner, Checkov, Scotty & Nimoy in TNG/Reboot films. Sarek & Amanda, too. Pike. And there have been plenty of crossovers in the episodes after the premieres.
1) TOS in TNG. Bones, Spock, Scotty, Sarek.
2) TNG in DS9. O'Brien, Picard, Worf, Keiko, L. Troi, Q.
3) TOS on DS9. Trials and Tribble-ations. Kang, Koloth, Kor. .
4) TNG in ENT. TATV.
5) TOS on VOY. Sulu. Rand. Kang. T'Pau.
6) TOS on ENT. Cochrane. T'Pau.
7) TOS on DISCO. Spock. Pick Mudd. Sarek Amanda. Una. Vina. Talosians? Guardian of Forever.
Egads, there are too many to list.
https://en.m.wikipedia.org/wiki/Star_Trek_crossovers