In Season 8 I forgot to claim the phenotype plasticity helm skin for my titan and warlock but I did on my hunter thinking it would just auto claim on the others (stupid, i know). Is there anyway to maybe get in contact with a Bungie employee and get these items sent to me or is that just not possible now? I just noticed the other day after taking a break and figured I'd post and ask.
Aloha! The other 2 answers are both half-correct. You are allowed to retroactively claim previous season rewards that were earned, but not claimed right away. However, that only goes as far as one season prior to the currently active season. Since Season of the Undying was 2 seasons ago, claiming those rewards is no longer available.