The change to the TXT registry introduced in #2157 breaks downward compatibility by retroactively limiting the maximum length of managed records. This means even with #2811 implemented, there will be cases where an upgrade from pre 0.12.0 to 0.12.0 will not be possible.

As adressed in #2816, the maximum length of a record is 63 characters. This also holds true for the registry TXT. To avoid CNAME conflicts, it is already necessary to use a prefix or suffix for the TXT records, limiting the maximum length of managed records to 62 characters in the best case (and even less if the suffix/prefix is longer than one character).

Adding the record type to the TXT record reduces the maximum length of the managed record to 63 - [prefix/suffix] - [record type]. This breaks downward compatibility with all setups with already existing records with longer record names than 63 - [prefix/suffix] - [record type], without an option for the user to change this behavior. If such records exist, creating the TXT record that includes the record type will fail.

Even without the whole backwards compatibility issue: #2157 adds a limiting factor that will pretty much seem random to end users, as the implementation results in a situation where for example A records can be longer than CNAME records.

Based on the above external-dns should provide an option to disable the creation of the record type TXT record or (if this is the only remaining TXT record type in the future) an option to disable adding the record type to the registry TXT record.

How to reproduce it:

Use external-dns < 0.12.0, use a single character txt-suffix (for example "-"), add a cname record with 62 characters (for example "thisisarecordwithareallyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyylongname.some.domain") and then upgrade to external-dns 0.12.0 and trigger a reconcile.

This should result in external-dns trying to create the TXT records "thisisarecordwithareallyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyylongname-.some.domain" and "cname-thisisarecordwithareallyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyylongname-.some.domain", with the latter failing as it viloates RFC1035.