We now have a final timeline and know the Collect versions in which changes will be made. ODK Collect v1.28 will be going out shortly (see the beta). It will show in settings that deviceID will change and that simSerial and subscriber ID will be removed in v1.29.
ODK Collect v1.29 will be out in Nov/Dec 2020 and will remove simSerial and subscriber ID. It will also switch deviceID to what we have been displaying as installID.
Sorry I missed your follow-up message, @Blitheringeejit.
Submissions will continue to be sent with the deviceID (ID in uppercase, I wrote it wrong above) query parameter. The value of that query parameter will become what is currently shown in the settings as install ID. It will look like collect:<16-char random string>.
I think you're exactly right. Additionally, users can currently revoke or never give Collect permissions to read their unique identifiers which can lead to blank values. Now the deviceID value will never be blank.
Thanks for bringing that up. I believe colons are safe to use in query parameters (see here, here), but I do see a number of questions about it. I'll do a little more reading around whether they should be URI-encoded. I've seen colons used as namespaced query parameters like we're doing and also as part of timestamps.