Many commenters are implying that there is a security issue here, and that I'm putting everyone in danger. That is quite frankly a pretty absurd claim to just casually make. I'm of course very curious to hear more details on what the security risk here actually would be?

Do you think I'm reading/writing sensitive data to/from subdomain-wide cookies?

Also, yes, the PSL is a great tool to mitigate (in practice eliminate) the problem of cross-domain cookies between mutually untrusting parties. But getting on that list is non-trivial and they (voluntary maintainers) even explicitly state that you can forget getting on there before your service is big enough.