Is it permissible to write "25 days remaining" or to count down the days to a specific event without linking that to "In shaa Allah (God willing)"?

It is recommended for someone who intends to do something in the future to say: "Insha'Allah (God willing)," seeking blessings and complying with the Almighty's saying: "And never say of anything, 'Indeed, I will do that tomorrow,' except [by adding], 'If Allah wills.'" This was frequently narrated from the Prophet, peace be upon him, as in the story of Sulaiman (Solomon), peace be upon him, who resolved to go around to his wives so that they would bear knights to fight in the way of Allah, but he did not say "Insha'Allah," so only one woman gave birth to half a man. Similarly, Musa (Moses) and the sacrificed son, peace be upon them, said it.

However, saying "Insha'Allah" is not obligatory, so there is no blame for omitting it. Blame is only for abandoning an obligation or committing a forbidden act.