iPhone 7 is ‘3rd’ tier now so you are right to question how long they will support it.
New iPhone SEs are going for about £250 if you can find stock which is a tier lower so £300 is a bit steep for a refurb. On the flip side the 7 is a much better phone.
That said they still support the 5S but I wouldn’t expect them to continue to do so in the next release. Rumour has it the 6 will also lose support next time round.