If you've noticed GSC impressions fall off a cliff recently, you're not alone:

• Most websites are seeing a significant drop in impressions since Google disallowed the num=100 parameter
• Google finally commented on this change, saying "The use of this URL parameter is not something that we formally support."

It seems like they're implying that since they never officially supported the parameter, they didn't really take anything away. Anyway, it's helpful to understand why impressions data changed in GSC in case you need to explain it to stakeholders.