I have a system of using month and year plus a common trailing string. My annoyance is that every year the last number on the year keeps moving.

Mine also along with multiple new pin codes for our phones every 3 months. Its annoying as hell and makes people just use easier and more memorable passwords rather than sticking with harder ones for longer.

If we take the EFF wordlist commonly used to generate passphrases, we have 7776 words. If we take ASCII alphanumerics and special characters we have about 92. Assuming we generate a NIST compliant password of 15 characters in length we get 92^15=2.8*10^29 of combinations. To get the same level of security (same number of combinations) using the EFF wordlist we need (log(2.8)+29)/log(7776)=29.45/3.9=7.6 around 8 words. If we assume the words are 7 characters long on average, plus the separators, we get 7*8+6=62 characters long. If we start to also include Unicode characters like other scripts or emojis (something I've recommended to others) the required number of words grows even more. I don't know about you, but I can't be bothered to type that all out every time, a password manager works for me.

Did they ever get into style? I use them, but I don't know anyone else who does. And some sites make diceware passphrases harder to use by e.g. requiring you to add numbers, special characters or capital letters, or by banning spaces. Or by banning non-ASCII, which makes it harder to use diceware passphrases in other languagess than English.