The only cast members from Season One to not appear in the final episode are Shannen Doherty (it is explained that Brenda couldn't make it due to prior engagements in London), Douglas Emerson (retired from acting after Season Two), Carol Potter and James Eckhouse.
Last show of the series.
Luke Perry's final performance as Dylan McKay.
Shannen Doherty didn't appear in the series finale because she declined to sign a release that would have allowed archival footage of Brenda Walsh to be used. That refusal of waiver is also the reason that Doherty's face was electronically scrambled when Brenda did appear in a scene used in the pre-finale retrospective show (the 1993 high school graduation).