There is an excellent article in the February 3, 2021 issue of World Politics Review (WPR) entitled The U.S. Military Must Be Nonpartisan, but Not Apolitical by Jared Taylor. The bottom line according to the author, who is an Army veteran, is that it should be incumbent on the military to avoid partisanship while affirming crucial political obligations during a crisis. Such as occurred January 6 in Washington, DC. I believe this also applies to military retirees as well.
If a retired soldier wants to write a letter to a local newspaper in order to advocate his or her personal political opinion(s), that’s fine. But don’t try to embellish it by signing off as retired military. Your writing should stand alone on its content, not anything else. If the letter is to the Army or Air Force Times, that’s different and those publications enjoy hearing from retired military.