Back in older editions of D&D there were rules for designing epic spells using seeds and factors. These epic spells still exist in Forgotten Realms currently (1400 DR) so what are the rules for epic spells in 5e? Are there official rules for this already made?
No official rules exist
There are currently no rules for creating your own spells. I checked the DMG, Sword Coast Adventurer's Guide, Volo's, and Xanathar's. If you want to use the old rules or spells created with the old rules you will have to sit down with your DM to convert them as homebrew material
According to the Epic Level Handbook from 3rd edition:
Epic Spells are considered 10th level for the purpose of Concentration checks, spell resistance and other determinations
The rule in 5e relating to spell levels is (PHB):
Every Spell has a level from 0 to 9
This rule is unequivocal.
As a result we can conclude that Epic Level spells no longer exist in 5th Edition.