Sure.I owe @NotAYakk a detailed response on the same topic, but this is only true if you both know what those saves are and have those spells prepared.
How hard is it to guess the low saves of a creature from even a cursory description or a tiny bit of knowledge?
A 30 foot tall giant isn't likely to have a super dex save. A golem or programmed automaton is probably going to not have a high intelligence. Etc.
Even barring common sense, if the DM doesn't hide their rolls, it is a round or two to find a weak save; and unless every foe is a unique snowflake, you'll figure things out pretty quickly.
That leaves a situation where a creature is misleading in its strengths and weaknesses and the DM hides all rolls and it is a unique snowflake never to be seen before or again. Then all you can do is try and presume misses align with stronger saves, or just use no-save spellcasting tactics (which still exist in 5e).
But at this point, you are making the spellcasters life as hard as you can. And that is possible! The DM can make everything occur in an anti-magic field.