It looks like this got broken between Maple 13.00 and 14.01.
If you really prefer that it not emit that error (instead of using :-combine, say) then you might consider patching it by adjusting the inert form of the procedure. For example,
IntegrationTools:-Combine( Int(sin(x),x)+Int(cos(x),x) );
| sin(x) + cos(x) dx
That is quite a crude use of subs, above, which apparently works out ok in my Maple 18.02 for this brief procedure. You ought to check whether you think it does what you intened. More robust and generally safe might be too get at the particular pieces for correction using op, check they are of the expected wrong form, then replace with subsop.
You would need to be extra careful about saving the modified module to archive, which can be tricky (and sometimes not practical) since doing it right can require first evaluating many other parts of the module. I don't suggest anyone who doesn't consider themselves expert try that. It's a lot simpler merely to have the fix-up be done as above, in one's initialization file.
Such changes would be better if accompanied by test examples. It's obviously risky to make changes to some command whose code is protected.