Yet, issue is there in 15.1.2

NO! it isn't fixed at all.
I still can't switch to text tool from (f.e.) eyedropper tool to text tool with "T" key.
Text tool activates for a second, then it returns to eyedropper tool.

It won't work also with other tools as first (preventing me from switching to text tool).

It's now 2020, and it is NOT fixed. Please do not sweep this under the carpet.

I had this infuriating problem too, but was able to overcome it entirely via following technique.
(Note: I'm using INDD 15.1.2 on Windows 10 in late August 2020).
Adobe should note that this is a workaround only and not a total solution to their bug.

In this thread there is a post by: Anonymous on May 29, 2020 13:49. which states:
"removing the Document Fonts folder (for a packaged indesign document) so no fonts were loading from the packaged Document Fonts folder. If even 1 font is still in the folder I get the problem, which also includes extreme lag when switching into or out of the text tool and selecting text."

Further to this, I then closed and re-opened document.

Hope this helps some people.

Just updated today and not fixed. Very very annoying.

Adding to the thread. Definitely not fix, and an extremely annoying bug to be working agaisnt.

A really time-consuming bug, please fix it.

August 2020, it's still happening when working with heavy documents or multiple open documents.

This wasn't fixed and is infuriating. I'm up date to date as well.

Still happening as of the latest update. If it's helpful, it hadn't been happening to me for quite a while and just came back with the new update released this week or last week? Nothing changed in terms of fonts being added, no new images or files. Working on a previous document that worked fine until today.

Why should we have to delete font folders that are needed? get this fixed Adobe!

Shame on you Adobe! So much invested and and frustrated artists. Fix the problem already!

When is this issue going to be fixed. it gets worse when you have more than one file open.

15.1.1 version still has this issue with the text and selection tool. 07-14-2020

At the top right, there is a gear icon. Click that and under Character, uncheck the Character Fonts option. This fixed it for me.

I can re-iterate what Anon said below:
"What worked: removing the Document Fonts folder (for a packaged indesign document) so no fonts were loading from the packaged Document Fonts folder. If even 1 font is still in the folder I get the problem, which also includes extreme lag when switching into or out of the text tool and selecting text."

Bizarre, but sure enough I was working in a file that was from a packaged ID folder. I already had the fonts installed so I could remove the Fonts folder from the Package with no problem. Now Type tool is selecting with the shortcut and staying selected :)

Still occurring on latest version.

Same issue under 15.1.1

Type tool Deselecting itself just reappeared in version 15.0.3. I just reverted back to 15.0.2 and bug resolved.

Additional info on this: I have both 14.0.3. and 15.0.3. I installed 15.0.3 specifically to fix this issue that is still occurring in 14.0.3. However, the issue is still present in 15.0.3 also.
Also tried clearing preferences (waste of time).
Also tried removing all installed user fonts from Font Book (Mac OS X 10.13.6).
What worked: removing the Document Fonts folder (for a packaged indesign document) so no fonts were loading from the packaged Document Fonts folder. If even 1 font is still in the folder I get the problem, which also includes extreme lag when switching into or out of the text tool and selecting text.