First of all, thank you all for being so patient and kind in providing triage and possible fixes.
Special thanks to @leoverde2, @mike-lloyd03 for the fix, and also @speelbarrow for providing a usable branch/fork.

We have released the 0.7.7 that contains the fix. It is now possible to use it with nvim 0.10+.

Remember that the current LSP implementation doesn't support all the features and you must ensure that you're not passing additional capabilities to your nvim config.

Example:

require("lspconfig")["arduino_language_server"].setup({
	cmd = {
		vim.fn.expand("~/.local/share/nvim/mason/bin/arduino-language-server"),
		"-clangd",
		"clangd",
		"-cli",
		"/usr/bin/arduino-cli",
		"-cli-config",
		vim.fn.expand("~/.arduino15/arduino-cli.yaml"),
		"-fqbn",
		"arduini:avr:uno",
	},
	-- capabilities = capabilities, <-- DO NOT specify that
})

Hopefully, when we have more bandwidth we will release an update to https://github.com/bugst/go-lsp to support more capabilities, thus avoiding crashes.

Thank you again and apologies for the huge delay 🙏