A substitute good for yoga classes is yoga videos on the web. The advantages of following a video over going to class are that you can do it at home, at any time that suits you, can skip or repeat parts, it is free, clothes are optional and the selection of teachers and classes is much larger. If you prefer to do yoga in a group, you can get some friends together, but this reduces the time flexibility and a large group needs a large room. The advantage of a yoga teacher is that they can correct your posture mistakes, either by telling you (doable in a video call based yoga class) or by physically moving your limbs to a different posture. A teacher may also be a commitment device – if you have trouble motivating yourself to exercise, then someone’s oversight may substitute for willpower.
Most yoga teachers seem to not use their advantage, meaning they just instruct in front of the class and don’t correct people’s postures. I have seen this in online yoga videos taken in actual classes and experienced it in most classes I have attended. I have tried about 30 yoga teachers overall.
Maybe some students do not want to be touched for physical posture correction or the yoga teacher has a phobia of touching, or (especially in the US) there is a fear of litigation related to any physical contact. Even telling someone about their wrong posture may be psychologically difficult (people may be taught that “if you have nothing good to say, don’t say it”), or a student not used to criticism may react negatively to it, however gentle and well-intentioned it is. Maybe the philosophy is “whatever is, is right” and that yoga is so individual that there is no such thing as wrong posture. I disagree on this, having sometimes caused myself slight injury by overextending joints in wrong postures (that went uncorrected).