> Where can I learn about which cryptocurrencies constitute securities and which don't? Is there a list?
Yes. Start with the SEC’s complaint against Coinbase: SOL, ADA, MATIC and ten others [1].
Beyond that, ask a securities lawyer or ask the SEC. Coinbase did. The SEC answered. Coinbase didn’t like the answer, and focussed its messaging on irrelevant statements (or a lack thereof) by Gensler about Bitcoin and Ether. If someone only cash traded those two, they would have a legitimate argument against the SEC’s ambiguity. But practically nobody does: instead they operate unlicensed and illegally-configured securities outfits.
The SEC has been super clear for most crypto except Bitcoin and Ether.