Sure, but what kind of protocol are you going to use to actually reap the benefits of these transient neuroplasticity effects, especially for learning something new like math or piano as an adult?

Same with ketamine. It's not like you can take it a couple of times and open some magic window where everything suddenly becomes easier to learn. From my personal experience and surface-level understanding of the current research, psychedelics(including ketamine) seem to temporarily relax hardened beliefs/priors and rigid neural pathways, which can help you see things from a fresh perspective. But learning something substantial like math or a new language after you've passed the most plastic stages of development is still going to take much longer than what these drugs can realistically help with.

They might make you see something differently or even get you extremely interested in it, but sustaining that interest and consolidating the knowledge or skill is still slow compared with childhood/adolescence. Of course, it depends a lot on what you're learning. For example, crystallized intelligence and sufficient motivation can make some things much easier to learn as an adult, but many useful things are also just boring to learn when you no longer have a childlike plastic brain or an environment built around constant learning.

Edit: Theoretically, you could accelerate learning by taking psychedelics/ketamine at a set frequency but it's a huge gamble because of their risk profile. eg. HPPD/trauma risk with classic psychedelics and bladder/neurotoxicity risk with ketamine if you get addicted or take it too frequently.