I know those from growing up with QBASIC and EDIT.COM, but I think the only software I've used that actually supports them is the Windows conhost (the default UI for command-line apps).

GNU/Linux[1] seems to support it. Windows probably does too.

[1] Tested on KDE Wayland and Xfce X11.

What? Ctrl/Shift Insert is supported basically everywhere in Windows, from the lowly Notepad to the behemoth that’s Microsoft Word. Browsers support it. Terminals support it. On Linux all Linux apps I’ve tried support it. Even Emacs which famously doesn’t support the usual C-c C-v supports it (mapped to kill-ring-save and yank). This is as universal as possible in the world without Cmd key.

In fact the last time I bought a keyboard I made sure the keyboard mapping can be customized so I can actually put the Insert key on it.

Same I grew up using Ctrl+Shift+Insert, and only really migrated over to Ctrl+C/Ctrl+V when I started using mouse-driven environments more heavily, because I could drive copy paste with my left hand and keep the mouse in my right.