How to paste without formatting on a Mac

Press Option-Shift-Command-V. In most Mac apps that pastes the text in the style of the text around it, without the fonts and colours it came with. To paste plain text from anything you copied earlier, or in an app without that shortcut, use Holdall: open its panel and press Shift-Return.

What macOS offers

Paste and Match Style: Option-Shift-Command-V, also in the Edit menu. It works in Apple's apps and many others. A few apps use their own shortcut, which their Edit menu shows.
Make the text plain first: paste into TextEdit, choose Format, Make Plain Text, then copy it again.

Both work only on what you copied last.

Fix it with Holdall

Download Holdall, drag it to Applications and open it.
Click Open Accessibility Settings and switch Holdall on, so it can paste into the app you're using.
Copy as you normally do. Holdall keeps each copy with its formatting.
When you want plain text, press Option-Command-V, move to any clip with the arrow keys and press Shift-Return. It pastes as plain text.

You can also right-click a clip in the panel and choose Paste as Plain Text. Holdall puts plain text on the clipboard before it pastes, so it works in any app that takes text. Return still pastes a clip with its formatting.

Text you paste often

Make a clip a snippet to paste it as plain text every time: right-click it and choose Make Snippet. Snippets can fill in today's date and time as they paste, which the guide on saved replies covers.

If it doesn't work

Nothing pastes. Holdall needs Accessibility to paste for you. Switch it on in System Settings, Privacy & Security, Accessibility. If it is on and Holdall still asks, click Reset and try again in its Settings, or run tccutil reset Accessibility ai.lfglabs.holdall in Terminal and switch it on again.