I didn't use VC++ 6 myself, but I'll take a guess at a few things:

  - IntelliSense

  - Tasks for external tools

  - Themeable

  - Extensible through plugins, for which there is a large ecosystem

  - Support for many languages, with syntax highlighting and linting (through plugins)

  - Support for different debuggers (e.g. debugging a Cordova app)

  - Support for different version control systems (TFS, Git, SVN...)

  - Minimap

  - Highlighting of all instances of selected text in the scrollbar (don't know if this feature has a name, but it's really useful)

  - Ability to edit multiple lines at the same time

  - Bracket matching

  - Code folding

  - Live, inline reference information

  - Not sure if 'go to definition' or 'go to implementation' was a thing in the days of VC++ 6?

  - Workspace zoom

  - Support for screen readers

  - Integrated terminal

It had most of those things, especially the important ones:

  - IntelliSense - Yes

  - Tasks for external tools - I think so

  - Themeable - Yes, back then you could theme the whole windows desktop and native apps would comply.

  - Extensible through plugins, for which there is a large ecosystem - Kind of, all the "plugins" came from MS.

  - Support for many languages, with syntax highlighting and linting (through plugins) - Yes, not sure how many.

  - Support for different debuggers (e.g. debugging a Cordova app) - Don't think so.

  - Support for different version control systems (TFS, Git, SVN...) - Not sure, it predates everything on that list.

  - Minimap - Don't think so.

  - Highlighting of all instances of selected text in the scrollbar (don't know if this feature has a name, but it's really useful) - Not sure

  - Ability to edit multiple lines at the same time - No

  - Bracket matching - Yes

  - Code folding - Yes (I think)

  - Live, inline reference information - Yes

  - Not sure if 'go to definition' or 'go to implementation' was a thing in the days of VC++ 6? - It was (not sure about goto implementation though)

  - Workspace zoom - Don't know what that is.

  - Support for screen readers - Probably via windows API's, but I'm not sure.

  - Integrated terminal - not sure.

Sure doesn't look like a lot of process in the intervening 20 years.

Huh, kind of impressed it could do so much!

Ah, I missed one important one though:

  - Cross-platform

By 'Workspace zoom', I mean you can dynamically change the zoom level of open files, without having to go through settings menues to increase/decrease the font size