The onboarding banner got removed from core: https://github.com/odoo/odoo/commit/92f56f1c398e1a25882d6b25ffd2e886f8cdfee2
You will either have to get used to a new way of visualizing your introduction steps (i.e. an empty Sales app has one (simplified) approach) or take a look at the various web.ListView qweb template inheritances for loading in an additional section.
Thank you so much Christoph!
That explains it perfectly — I wasn’t aware the onboarding panel was fully removed from core in v18. I’ll explore the alternative approach you suggested. Appreciate your help!