This is probably a dumb question but that never stopped me before.
My computer currently is set up to dual boot Linux and/or Windows, and I am considering deleting windows altogether.
My question is when Linux goes on the hard drive will it boot with efi by default or the old way?
I gotta tell ya that the efi stuff and grub gives me the heebie jeebies because it is so easy to make a mistake that can be catastrophic. I can tell you stories.
So if I were to wipe the disk completely and install, say Ubuntu (or whatever) will bootloader use efi by default.

ESP (EFI) partition is separate from Windows partition, just make sure you leave it intact.

Edit: If you wipe the drive then you have the choice, EFI install or legacy. You set the method in the BIOS setup.

Short answer, yes.
Unless you change it in the firmware - I've been advising people to not use CSM (legacy mode) for years. There have been some flaky firmware implementations from the hardware vendors - I would delete everything other than the EFI rather than erase the disk. Just in case.

If your computer is currently set for secure boot uefi then it will stay that way if you format disk. There have been some reports of some systems keeping secure boot keys on the disk. I've never seen it. Guess you could search your system for that information. I install a lot of uefi systems.

In a very rare case I'd think you may have to access efishell to fix something.

There are some differences in install media in some cases. Some people create a legacy usb and can't figure out why it won't install to uefi.

Before you go too far. Check command efibootmgr. Some bios add entries and won't stop till it crashes.