Envoyer ou renvoyer could be irregular because they are spelt j’envoie ou je renvoie in the present tense: the y is replaced by an i.
All the verbs in -oyer must be deemed irregular then: dévoyer, chatoyer, soudoyer etc..

The definition of “irregularity” is not really fixed, it’s kind of a pedagogical concept in my opinion.

“Envoyer” and “renvoyer” are likely mentioned as special cases because they have future stems that are distinct from their infinitive stems and that cannot be derived from them by any standard rule of French conjugation: “enverr-” and “renverr-“. I don’t know of any other -er verbs with multiple stems like this that cannot be derived via one of the standard stem-change rules (“aller” of course has a future stem that is entirely unpredictable from its other stems, but its present-tense conjugation is enough on its own to show that it is irregular in other ways than stem changes.)

The alternation between “e” or “é” and “è” may be considered regular because in standard French, it’s phonologically impossible to have a sound like /ə/ or /e/ before a consonant followed by word-final “mute e”. A French word can’t have a pronunciation like */ləv(ə)/ or */sed(ə)/.

It is thus phonologically necessary for /e/ or /ə/ to be replaced with /ɛ/ in this context, and the standard way to represent the sound /ɛ/ in this position in French orthography is either “è” + consonant letter + “e” or “e” + double consonant letter + “e”.

In the future and conditional of verbs like céder, the vowel does not come before word-final “mute e”, so phonologically it is not so simple to predict whether /e/ will be replaced with /ɛ/. In fact, the use of /e/ vs. /ɛ/ in non-final syllables is rather variable/complicated in French, so the future of verbs like “considérer” can be written either as “considérera” etc. or “considèrera” (compare “événement/évènement”). But verbs like “lever” always are written with “è” in the future and conditional.

According to an article by ChloeF on FluentU, French verbs with infinitives in -oyer and -uyer always change to -oi- and -ui- before mute e (including in the future and conditional forms for verbs other than envoyer and renvoyer), while verbs with infinitives in -ayer (see the question on this site about essayer) may or may not change to -ai- : it’s optional.