Using the '/effect give @s <effect>' command in 1.13 onward will show you "[<seconds>] in the command suggestion, what is correct for almost all effects, but:
The effect minecraft:saturation as well as minecraft:instant_damage/health will give you the effect in ticks instead, but the command suggestion still tells you to enter duration in seconds.

>Therefore, if I type '/effect give @s minecraft:saturation' I have to enter "seconds" next, the effect will be given in ticks, I would need to enter 20 in order to apply the effect for a full second. Yet I am still told to enter the effect duration in second, if you don't know the effect will apply in ticks and not in seconds, contrary to what is told you, it causes some confusion.

 

And if we are already on the subject...
Why does the _/effect command give effects in seconds and not ticks, if I summon an entity with  the NBT data '{ActiveEffects:[\{Id:23b,Amplifier:1b,Duration:1}]}' the effect will be given for 1 tick because Duration refers to the Duration in ticks. 
The '_/time set' command gives you the option to choose between ticks, second or days, having a similar system for effects would certainly be a nice quality of life addition to the game, in my opinion. Example: '_/effect give @s minecraft:levitation 4t 75 true' = gives you levitation for _4 ticks without showing particles. 

But changing the command suggestion if you try to give you saturation, inst. dam./health to make it say "ticks" instead of second would at least clear up some confusion. 
But if you already fix it, then why not allow players to give all effects for less than a full second with the /effect command, I can't imagine it being hard to do and the game will give you the effect in ticks anyways, just in 20 ticks at a time, but then it counts down the effect timer every tick, so if you use the /effect command the game will first multiply your time input by 20 and give you the effect in ticks anyways? 

And let's be honest, if you want to give a player an effect that isn't saturation or inst. dam./hp you have to either summon an Area Effect Cloud, less than optimal, it involves entities and can potentially give the effect to more entities or not give it to the player you want to give it to, or you have to clear the effect for the player after a certain amount of time what also sucks because it involves multiple commands. 

Anyway, hope this gets fixed, love the game, thanks for everything <3