I doubt it explains any reasonable fraction of this, but github moving from early adopter techies to general population "normies" would be a reason for the shift. I would expect it explains at least some increase in the use of em-dashes.
You can remove em dashes from the analysis and the trend is still there: newly created accounts are still 6X more likely to use the remaining LLM indicators (arrows and bullets, p = 0.00027).