If she had such very strict boundaries during therapy, I think it's safe to assume she may just not keep in touch with clients at all after termination. I think the safest would be to ask the therapist on the last session what their policies are about post-termination contact, otherwise you could try calling and asking about it.

There's one other aspect I haven't seen in the responses so far. I mean, she has cancer and is undergoing chemo treatment, right? She may feel weak and overwhelmed with her own emotions. I think it's a sign of great strength that she can keep working as a therapist even while facing all this. It's not emotional just for you but also for her and the ways this must be affecting her whole life are a reason in and of themselves not to react so quickly, especially to a letter coming from a former client. There are many possible reasons why she hasn't replied yet and maybe won't (though maybe she will). Some reasons may have to do with you being a former client and I imagine there might also be reasons related to her being a human being dealing with an awful illness.

I realize this must be extremely difficult for you too. I imagine this therapist has been like a safe place for you for the past 13 years and that's a lot of time and perhaps a lot of trust. You have invested a lot in this therapy even if you won't call it a relationship. No, I don't think it's too much at all to expect some form of feedback from your former therapist after pouring your heart out and after all the courage it took you after all these years. Just realize though that you may or may not get it - maybe she needs to process things first so she won't give you a rushed answer or maybe she just isn't going to reply at all. If it's important to you, the safest bet would be to take initiative, call her and ask about it, perhaps have a few more termination sessions. Some therapists advise their clients to have 2 to 4 termination sessions after a much shorter time of working together, exactly because it can be so complex to leave, and in your and your former therapist's situation perhaps even more so.

Or perhaps you might like to wait until the holidays are over, see if maybe you get a response in the meantime and also get a chance to talk to your new therapist before deciding on your next move (if any).

Anyway, I wish you the best in your therapeutic journey with your new therapist!