##Provide special quests which result in access to such magic

Monsters in 5e often possess abilities that PCs cannot replicate without their assistance. We even have examples in the Monster Manual related to permanent magical effects: Demonic infestations often permanently mark the land around them, and particularly powerful demons leave a permanent shadow behind when destroyed (so PCs might kidnap, bind, and sacrifice a powerful demon to render a shield or other suitable object or collection of objects permanently cloaked in shadow), the Drow mass produce magic weapons and armor with a unique 'decays in sunlight' drawback (so PCs might engage in magical disguise and political intrigue to gain entrance to a drow college of magic, so as to steal the secret to their seemingly exceptionally-low-cost crafting process), and the Galeb Duhr are somehow bound to the Material Plane so that, unlike other elementals, they do not return to that Plane when they die but instead remain on the Material (so PCs might seek one out and study its connection so as to learn how to similarly bind themselves).

Providing additional monsters, ancient magical traditions, or other sources of power outside of the abilities players can access via class progression forces players to quest for the abilities needed to make a particular effect permanent, which works well. It is important to remember, however, that players can assume the powers of most any creature via high-level polymorph effects, and thus permanent magical abilities should only be granted to monsters in such a manner that the ability itself would still require significant adventuring, otherwise the magic should be tied to knowledge, skill, culture, or Providence such as to require the players to actually encounter real members of that race.

Additionally, rather than having an effect be permanent until dispelled, consider having an effect be tied to some conditions external to the PCs, and then letting the PCs concoct some way to render such an effect permanent. This seems to be more in keeping with 5e's approach to monster-based magic-- particularism trumps universalism. An example of this in core D&D 5e would by the Myconoids' Rapport Spore abilities. These spores allow telepathic communication for up to one hour per dose, and an adult myconoid exudes a permanent radius of such spores. PCs have access to all manner of magical and mundane spore preservation techniques, and so could conceivably gain what would effectively be Rary's Telepathic Bond at-will except limited to 30ft communication.

Provide special quests which result in access to such magic

Monsters in 5e often possess abilities that PCs cannot replicate without their assistance. We even have examples in the Monster Manual related to permanent magical effects: Demonic infestations often permanently mark the land around them, and particularly powerful demons leave a permanent shadow behind when destroyed (so PCs might kidnap, bind, and sacrifice a powerful demon to render a shield or other suitable object or collection of objects permanently cloaked in shadow), the Drow mass produce magic weapons and armor with a unique 'decays in sunlight' drawback (so PCs might engage in magical disguise and political intrigue to gain entrance to a drow college of magic, so as to steal the secret to their seemingly exceptionally-low-cost crafting process), and the Galeb Duhr are somehow bound to the Material Plane so that, unlike other elementals, they do not return to that Plane when they die but instead remain on the Material (so PCs might seek one out and study its connection so as to learn how to similarly bind themselves).

Providing additional monsters, ancient magical traditions, or other sources of power outside of the abilities players can access via class progression forces players to quest for the abilities needed to make a particular effect permanent, which works well. It is important to remember, however, that players can assume the powers of most any creature via high-level polymorph effects, and thus permanent magical abilities should only be granted to monsters in such a manner that the ability itself would still require significant adventuring, otherwise the magic should be tied to knowledge, skill, culture, or Providence such as to require the players to actually encounter real members of that race.

Additionally, rather than having an effect be permanent until dispelled, consider having an effect be tied to some conditions external to the PCs, and then letting the PCs concoct some way to render such an effect permanent. This seems to be more in keeping with 5e's approach to monster-based magic-- particularism trumps universalism. An example of this in core D&D 5e would by the Myconoids' Rapport Spore abilities. These spores allow telepathic communication for up to one hour per dose, and an adult myconoid exudes a permanent radius of such spores. PCs have access to all manner of magical and mundane spore preservation techniques, and so could conceivably gain what would effectively be Rary's Telepathic Bond at-will except limited to 30ft communication.