I am seeing the same thing in the Finder on Catalina. My output is the same:

🦊 qlmanage -m plugins | grep TextMate
  public.source-code -> /Applications/TextMate.app/Contents/Library/QuickLook/TextMateQL.qlgenerator (1.0)
  public.text -> /Applications/TextMate.app/Contents/Library/QuickLook/TextMateQL.qlgenerator (1.0)

Off topic (apologies in advance if I am stating the obvious):
When a message is being composed as rich text (depends on your email client settings; but I believe the default is to reply in the same format is as them message that was received); with a simple copy and paste (⌘ + V) the OS carries the style information and pastes the contents using that formatting. This is what you are seeing. If you don’t want the formatting to happen you paste the content using the “Paste and Match Style” option in the Edit menu (⌥ + ⇧ +⌘ + V).

Best,
Farhan

On May 31, 2020, at 8:00 AM, textmate-request@lists.macromates.com wrote:

From: "Marc Wilson" <posguy99@gmail.com>
Subject: [TxMt] Re: TM's QuickLook plugin
Date: May 30, 2020 at 12:18:26 PM EDT
To: "TextMate Mailing List" <textmate@lists.macromates.com>
Reply-To: TextMate users <textmate@lists.macromates.com>


No, just Finder here.  Well, Finder and Forklift, but Forklift is not replacing anything, it’s just the OFM I use.

I’ve been poking around and TM’s QL generator on Catalina is not being registered for the same filetypes that it is on Mojave.  I’ve collected a bunch of resources and am trying to make time to see if I can do some investigation.

Can I ask you, what filetypes does your Catalina install have registered to the TM plugin?

[ 9:16 AM][ttys000][/Users/mwilson]
mbp13 $ qlmanage -m plugins | grep TextMate
  public.source-code -> /Applications/TextMate.app/Contents/Library/QuickLook/TextMateQL.qlgenerator (1.0)
[ 9:16 AM][ttys000][/Users/mwilson]

(unrelated side comment… wow, that was weird… I didn’t expect a c-n-p from Terminal into my email client to bring the Terminal theme with it)

--
Marc Wilson