People do pay for XP boosts. However given how plentiful they were in the BP itself and the fact that you get an easy 1000 XP a day by playing 6 games it’s not like you NEED to buy them to get anywhere. I finished the BP before New Year.
Events cosmetics are not locked behind paywalls.
Weekly Ultimates aren’t Paywalled either (although they mostly are quite underwhelming and not very “Ultimate”). Sure, people pay for Challenge Swaps but again - there is no need to - since running out from the one’s in the BP I’ve been doing without Swaps.
All that being said: the majority of Cosmetics is paid for (store) or paywalled (BP Premium). It’s still quite for off from “EVERY single cosmetic”.