Telnet is an internal development interface and not a function of the FRITZ!Box. Enabling Telnet enables an interface that can be used to make extensive and sometimes even harmful modifications to the FRITZ!Box. For this reason, we did not and do not support FRITZ!Boxes with Telnet enabled. The note "Changes not supported by manufacturer" on the "Overview" page points this out. You can only remove this note if you run the recovery tool for FRITZ!OS, which also deletes all of the FRITZ!Box settings.
Starting with FRITZ!OS 6.25, Telnet is no longer supported and also cannot be installed accidentally.

AVM is open for FRITZ!Box users who actively participate in product development and also develop their own products (for example freetz). We also did not and do not prevent users from installing custom-designed firmware if they really want to do so.