I am user of Kdevelop on fedora 34 and a longtime user of Kdevelop. With the most recent update using dnf, Kdevelop does not open .html files so that they can be edited. Other files tried, (.e.g. .js, .txt, sh) are editable
There is a lock icon at the bottom left which only appears on .html files. I have checked file permissions and they are all highly permissive.
And I have googled extensive kdevelop html lock, kdevelop lock icon, etc all without any meaningful hits.

PLEASE, provide some guidance on how to make the file editable.

and some documentation references on how KDevelop works, what editor it is based on. Thanks in advance

Here is the answer (from Friderich) that solved this issue:
Not sure exactly if this is your problem, but recently someone with a similar
update history hit a similar problem, which could be solved this way:

Please check if you have some file
~/.local/share/mime/packages/user-extension-html.xml
around. If so, remove it, this is a file for some old hack to register the
default browser, using a custom MIME type. And which seems now to be preferred
over other MIME type definitions of files with the extension ".html". But
sadly lacks the information that HTML files are a subtype of plain text files,
thus KDevelop does not try to load a text editor, but instead tries to load
whatever KPart is chosen as preferred MIME type handler for

Once you have removed that file, run
update-mime-database ~/.local/share/mime
so the actual MIME database (generated from those files in mime/packages) used
(indirectly) by KDevelop is up-to-date and has the normal definition of HTML
files again, which also defines them as subclass of plain text.

You might need to restart KDevelop first though, perhaps some data is also
cached in the runtime instance.

Did this solve your issue?
YES!!!