Digital cable length- SPDIF vs AES/EBU- 1.5 meter for either?

Can someone direct me where I can learn why the 1.5 meter length is important? 

I got a 50 feet AES cable run (using 4 out of 8 channels), it’s freaking perfection, it's the best audio interface by miles until wireless Dante becomes more available.

Pro audio equipment rocks. (Only downside is I’m still selling off my high end audiophile equipments to this day...)

Thank you and indeed it seems reasonable as a minimum length.

I remember the original threads and article claiming this and had bounced it off a very sharp engineer friend of mine at the time and several others. In short, I don't believe it is true; there are more than a few high-quality SPDIF and AES cables on the market which test out perfectly at 1.0 meter.