I inherited my investments but in the year 2015...do I still say they were inherited on my 2020 tax return

[DoninGA]

No, not on the 2020 tax return.

[Critter-3]

If the 1099-B doesn't give a date then you will continue to use the INHERITED option  or simply enter the DOD ... either will get you a long term situation.  

[Opus 17]

An inherited IRA is always an inherited IRA and the rules on withdrawals are always different than in a regular IRA.

Regarding regular investments (stocks, bonds) you should only have anything to report when you sell them (on a 1099-B form).  When you do sell, you will still need to list them as inherited, and the date, and the value on the day you inherited them, to calculate your capital gains.  (Your broker is probably tracking this for you.)