If the Yadava clan during Krishna's time became extinct because of the curse of Gandhari, then who are the Yadavas now? How can they claim to be descendants of the Yadavas of Mahabharatha?

First of all not all the Vrishnis (the line of Krishna among the Yadavas) were killed. Vajranabh was still unborn and hence was unaffected by the curse. There are lines of Vrishnis and Andhakas still alive but they don’t use the Yadav name, which is a 20th century phenomenon. Bhatis (from Yadupati), Jadons (Jadu/Yadu (vanshi)), Jadejas (Yaduja), Chudasamas, they all belong to that line however their names come from individual kings for better compartmentalisation and not the overall Yaduvanshi ambit.

Secondly the Yadavas of today are Ahirs that took that name through the campaign of upward mobility of lower classes that was run by Arya Samaj. The culprit of the act being Krishnanand Saraswati and Rao balbir Singh of Rewari, the adoptive son of the adoptive son of Tula Ram.

It is interesting to note that in Mahabharata 16.7 (Mausala Parva, Chapter 7) the Abhiras (older name of the Ahirs) attack the returning Yadava cavalcade in the aftermath of Krishna’s death and the fall of Dwarka and abduct and “violate” Yadava women. The children born of this act are called Yaduvanshi Ahirs, they are fewer in numbers than the claimants to this name. Their Gotras are Aphariyas, Kausala and a few others.

Here is the text from Mahabharata.

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They held a Mahasabha in Allahabad in 1910 from whence the cow herders across north Indian plains and some in central India took the Yadav name. There was a Janeu Andolanas well, headed by Arya Samaj missionary Krishnanand Saraswati (who was himself an Ahir before the name change) where they put the sacred thread over the lower castes and gave them names and heritage of high born lines of Kshatriyas and Brahmins.

It is quite comprehensively documented by British historians of that era, I suggest that you read Sons of Krishna by Lucia Michelutti. She documents how the rise of Yadav claim, even Yaduvanshi claim rose from around 400 odd individuals in 1910 to more than 65000 in Mainpuri alone.

The Brits wanted more men to fight in WW1, and they therefore officiated this change.

They were Shudras and hence still fall under OBC.

PS: The Ahir population of UP-Bihar (Eg. Mulayam Singh Yadav, Lalu Yadav) is not even Ahir, which is why the actual Ahirs of Ahirwal in Rajasthan and Haryana never marry among them.