Are you required to declare past conflicts of interest?
Usually the consideration is not about the funding received when the paper was published, but received while the work was being done. To illustrate: if a grant is won after a paper is submitted, but before it is published, it would be strange to see it declared as a conflict of interest.

But if a researcher persistently receives funding from a particular source, and then takes time off from that funding and uses those windows in funding to publish material in support of that source, then it would be inappropriate to stay silent about that funding: doing so would be against at least the spirit of the rules about declaring conflicts of interest; and may be explicity against the letter of the rules, when the papers contained work that was done during the funding period.