I've been racking my brain to come up with something to replace the word "humanity" in a fantasy setting to encompass all sapients.
Has anyone got anything that works well as a replacement in these phrases;

"oh the humanity"

"for all mankind"

"not allow humanity to come to harm"

It may include non-sapients too, but the other commonly used term tends to be "creation."  i.e.  "For all of creation" : "not allow creation to come to harm".

It doesn't quite work for "Oh the humanity".  Perhaps " -the people!".  Superman once said it to Zod in the Christopher Reeve 2nd movie, but it was in reference to specifically humans then.

Don't replace "humanity".  Focus on rephrasing the the idea or concept you want to express.  In some instances simple words like "decency" or "beings" might work; but overall the existing phrase we use in reality is based a limited set of conditions when compared to the the broader universe that can be applied within the game.  

So "for all mankind" may be restructured to "for all the citizens of Faerûn" or "to benefit the children of the Gods" or "in accordance with laws and principles of life".  

In short, you have to create unique language usage: expressions, slangs, philosophical phrases, idioms, etc.   Don't assume it will always be as easy as changing one word and be open to it being an evolving process where you may learn that a term you use may not be inclusive and thus you adjust.  

I'm reading  book right now where instead of "mankind" they just refer to sapients as "kynd", so like "be you kynd?" or "for all kynd" are how they would say such things. 

“Mortalkind” is an easy substitute for mankind.  Even your most long-lived species, like elves, are mortal, so this catch-all phrase covers most playable species. There are some edge cases - Warforged have no set lifespan and Reborn could be functionally immortal depending on the nature of their existence - but those are all going to be species option where the question of “am I really part of mortalkind” is an existential question that can be a fundamental part of the character, if the player is so inclined.

For a formal usage of “humanity” - such as your third example - you can probably use some variation on mortal as well.  “Not allow mortalkind to come to harm” is a perfectly fine restating of the Zeroeth Law for a world where humans are not the only adventurers.

For an exclamation, such as your third example, I would avoid doing anything fancy.  Your best option might be avoiding that particular phrasing in its entirety.  Exclamations are a bit different in their cadence - to have an effect, they should roll off the tongue comfortably and with a certain degree of punch.  You do not want your delivery or the reception of an exclamation mired by the mental gymnastics needed  to wrap around a bit of an awkward phrasing. 

Depending on how religion works in your world, it might also be appropriate to say "For all that is holy", as life= holy/ sacred. 

"Light Born" could also summarize "good god created". 

I've seen "Kith and Kin" being used, I think in...Pillars of Eternity? Not sure... 

Quote from Shepherd_Hubbard >>

It may include non-sapients too, but the other commonly used term tends to be "creation."  i.e.  "For all of creation" : "not allow creation to come to harm".

It doesn't quite work for "Oh the humanity".  Perhaps " -the people!".  Superman once said it to Zod in the Christopher Reeve 2nd movie, but it was in reference to specifically humans then.

Quote from Caerwyn_Glyndwr >>

“Mortalkind” is an easy substitute for mankind.  Even your most long-lived species, like elves, are mortal, so this catch-all phrase covers most playable species. There are some edge cases - Warforged have no set lifespan and Reborn could be functionally immortal depending on the nature of their existence - but those are all going to be species option where the question of “am I really part of mortalkind” is an existential question that can be a fundamental part of the character, if the player is so inclined.

For a formal usage of “humanity” - such as your third example - you can probably use some variation on mortal as well.  “Not allow mortalkind to come to harm” is a perfectly fine restating of the Zeroeth Law for a world where humans are not the only adventurers.

For an exclamation, such as your third example, I would avoid doing anything fancy.  Your best option might be avoiding that particular phrasing in its entirety.  Exclamations are a bit different in their cadence - to have an effect, they should roll off the tongue comfortably and with a certain degree of punch.  You do not want your delivery or the reception of an exclamation mired by the mental gymnastics needed  to wrap around a bit of an awkward phrasing. 

 +1 to these, "all creation" and "all mortalkind" seem to be terms that a varied non-humanocentric setting would use.

Now, the term everyone in D&D should really be up in arms about is "humanoid" 😁

Makes me think of the Star Trek opening speech change, particularly that scene at the end of Undiscovered Country where Kirk says, "where no man, no one, has gone before."

And borrowing from Star Trek you could use "sentient species." Has a bit of a Sci-Fi tinge to it but since WotC intends on moving from "races" to "species" it could follow that nomenclature if your group is so inclined.

Could also do something like X-folk. For example if your campaign takes place in a region called the Berrywold you could could collectively call the various races there the Berryfolk. Also allows you to have common suffixes if you're doing something like uniting countries within a region to be like, "The Seafolk are a threat to all of us. Though you are a Hillfolk and we are Valleyfolk, together we are the Berryfolk."

Same idea if you go with "kin."

Quote from VitusW >>

And borrowing from Star Trek you could use "sentient species." Has a bit of a Sci-Fi tinge to it but since WotC intends on moving from "races" to "species" it could follow that nomenclature if your group is so inclined.

 
Sentient wouldn't be accurate. Sapient is.

Sentient: may be able to have emotions, a sense of personality and preference. Many creatures in real world have this. Even cats and dogs and mice.

Sapient: self-awareness and understanding. In real world, humans are the only confirmed sapient creatures. It's why we're called homo sapiens.