Chain selection has recently been discussed in this thread: Offer a new endpoint (and ACME spec update) to list available chains
As I understand this proposal, it will not directly affect chains. Whether you're getting the RX or EX chain depends on the key type you submit. The profile doesn't affect this. Remember that the allowlist for ECDSA will eventually be removed. Profiles don't sound like a drop-in replacement for it.
In the future, the profile could avoid having to allowlist more features. For example, instead of having accounts allow-listed for "reduced set of EKU", this can be controlled via the proposed "modern" vs "legacy" profile.
I can imagine scenarios where the profile does affect chains (e.g. there's a new hierarchy with completely different compatibility not selectable by key type). But in the general case, the profile shouldn't affect key types IMHO.