well, in theory, this might have been a speech issue, but only if the government had stepped in and shut it down. had that been the case, and the company argued against it, i would be standing up for the company.
as it is, the board is owned by a private company that took it down on it's own prerogative. as such, it's a property rights issue, not a speech issue - and perhaps a contract law issue, if the purchasing party feels an agreement was breached.
i would hope that bernier and his party were properly refunded. that is the only meaningful legal issue at hand.
but, people arguing that it's not a speech issue because of the content of the ad are wrong on their face, as well.
https://www.cbc.ca/radio/thecurrent/the-current-for-august-27-2019-1.5260356/removing-anti-immigration-billboards-is-censorship-says-columnist-1.5260365