According to this Oxford dictionary article, both are correct. "Slew" is a collective noun, like "team" or "group". And in general:
[T]he verb form used can depend on the emphasis of the sentence, and accepted regional usage, so no wonder many people are confused. In British English it’s absolutely fine to treat most collective nouns as either singular or plural – you can say my husband's family is very religious or my husband's family are very religious.
[...] in American English you are much more likely to see, for example:
His company's legal team is investigating the matter.
His company's legal team are investigating the matter.
So both "is" and "are" are acceptable here.