Use same private key for multiple bitcoin-based coins, good idea?

[epsi1on]

How about Electrum for LTC[1]? It's actively developed and can be used as offline/cold wallet.

Didn't test it yet. not sure if it support

Be careful if you really intend to only backup HEX private key. You''ll need to remember what kind of address you use and whether it use compressed/uncompressed key. Otherwise you'll be forced to try generate all possible combination.

yes,  I finally ended up writing assest and first few characters of each address on the same paper. also hex is not easy to read and write, with a single mistake all are gone wrong... this is some time saw the mnemonic, and HD wallet, but i thought it is only for bitcoin. which was not true and it can be used for many coins...
Writing down mnemonic phrase is much simpler than writing down hex numbers. But on the other side, anyone on earth can identify a paper with mnemonic phrase on that as crypto key, but a hex writing is less identify-able as private key

Anyways the mnemonic and BIP44 etc. is how some bitcoiners here do keep their asset, so i definitely rather to test it.

[o_e_l_e_o]

this is some time saw the mnemonic, and HD wallet, but i thought it is only for bitcoin. which was not true and it can be used for many coins...

A seed phrase simply gives you a starting point to deterministically generate random numbers. What you then do with those random numbers is up to you. You can use them as a private key for any coin which requires a numerical private key (which I assume is every coin, but no doubt there is some shitcoin out there which doesn't use private keys or something weird ).

But on the other side, anyone on earth can identify a paper with mnemonic phrase on that as crypto key, but a hex writing is less identify-able as private key

I think that's an unnecessary concern. If an attacker is targeting you because they are specifically looking for bitcoin or other crypto, then it doesn't matter if they find a seed phrase or a private key - they will recognize both and steal your funds either way. Similarly, if you are subject to an opportunistic attack by someone who does not know what bitcoin is, then a seed phrase or a private key will both look meaningless to them. And the security of your coins shouldn't depend on hoping that an attacker does not recognize what they found, but rather ensuring that it is never found, or even if it is, it can't be used to steal your coins. The most straightforward way to do this is via a seed phrase with an additional passphrase backed up separately.