We can look at it from
How to make a Song Timeless.
Here's my take
*Lyrics : universal themes that applies to "anytime anywhere" (thus No political, cultural, technology, clothing, styling, name dropping).
Inverted commas because Nothing applies to anytime anywhere.
*Instrumentation : sticking to those that anyone at anytime now future past used, will use.
This means mostly Acoustic only : guitar, piano, flute, mouth organ, strings.
Nothing electronic except electric Guitar.
*Composition : we all write compose according to a genre(s).
We can leave out specific aspects that were prevalent in each decade.
And just focus on what was timeless across every decade.
*Production : Not the in thing in production whatever it maybe such as auto tune.
Not the use of this fx and that fx.
*Drums. Some genres Drums are a must.
Every decades drums records had the sound of that decade (generally).
It's not the groove. It's the way they were recorded, produced (generally).
Two notable exceptions are the Drums from James Brown, and the Winston's Amen Brother song known as the AmenBreak :
estimated used total 10k times on music across several genres even upto now.
As for Electronic drums : listening to 80s,90s 2k songs we can date them according to the programmed grooves and sounds from drum machines of the era.
Although lots still like using vintage drum machines.
Thus Acoustic and Electronic Drums trying to find those timeless grooves, timeless production.
If the genre of music doesn't require drums, then leaving drums out.
As for Instrumental music.
If sticking to acoustic instruments or samples of acoustic instruments, samples of nature should be straightforward.
It's pretty easy to tell apart decades in Electronic music because of Synthesizer development, development in Styles.
I would say we are on very safe ground composing Experimental Electronica.
1940s, 50s, 60s Experimental Electronica sounds same as current Experimental Electronica and vise versa.
I would say we are on relative safe ground composing Ambient.
1970s Ambient can sound as 2020 Ambient and vise versa because of the basic structure and basic instrumentation of Ambient.
It's the one genre in Electronic music that's the easiest to do.
I would say much of 1970s Instrumental Electronica sounds timeless :
so using just Analogue Synths, similar Styles as those 1970s Instrumental Electronica.
What makes a track sound old?
For everything after the recording stage: hardware/software and how you use it.