Clorox seems like a cruel thing to put on wood.
I imagine that you'll feel very upset then by the thought of using lye on wood, but that too is widely done :-)
It feels "dryer" or something.
That may be salt residue. Briskly brushing the surface may remove this and return the wood to feeling like it did prior to bleaching.
Is doing this a mistake?
With two-part wood bleaches (A/B type) one bottle contains sodium hydroxide, lye, which is very basic and likely in a more concentrated solution than common bleach. In the traditional fuming technique to darken oak the wood is exposed to the vapours from concentrated ammonia, another strong base/alkali.
In reality wood is pretty tough stuff, and for many woods even heavy treatment with caustic chemicals — as is done in some furniture-stripping operations — does little more than slight damage to the surface wood fibres only (somewhat similar to some exposure to sunlight and rain).
What is the long term downside to bleaching poplar with chlorine bleach before staining it?
None that I'm aware of.
Some people like to neutralise the bleach by wiping down with vinegar (safety note: after the wood has dried, do not do this while the bleach is still liquid) but that is likely a pointless step given a number of other treatments with alkaline chemicals are not followed by any use of acids.
From the opposite direction, some users when they are treating wood with vinegar or other acids like to neutralise these with a dilute solution of baking soda or washing soda (both mild bases). But this too is unnecessary, and in addition can have unintended effects on some species, changing the colour of the wood slightly.
In reality most wood species are naturally acidic, so the surface wood fibres being left acidic won't matter to them. And where an alkaline chemical was used and there's any excess (un-reacted) chemical in the surface wood fibres it will be neutralised naturally over time.
I have some oxalic acid coming too that I'm going to play with.
Oxalic acid is the wood bleach of choice for dealing with water staining and stains formed from iron reacting with the tannins in wood. It can also be used to treat some of the discolouration from a burn or scorch.
Note that oxalic acid is quite toxic, so do follow safety instructions. Wearing goggles, a dust mask and gloves is advisable when handling the dry powder as well as the prepared liquid solution.
Unrelated to your question, more as a headsup for other readers:
Generous coat of minwax pre stain conditioner. Leave for 30 minutes without wiping excess (it all soaks in).
Firstly, it appears that "pre-stain conditioners" consistently have incorrect instructions. According to Bob Flexner you tend to get far better, more consistent results if you wait for the product to dry rather than applying stain a short while later as it appears you did here. To say that a lot of user experience supports this would be an understatement, as "pre-stain conditioner" is nothing more than a dilute finish in most cases; and very dilute shellac or varnish*, thinned protein-based glues also, have been used for centuries to do the very same job — what used to and still should be called sizing.
Secondly, conditioners should generally be used only where needed: for controlling blotching in blotch-prone woods. It's a needless expense and time penalty otherwise, and in addition it can negatively affect the very results you're hoping to achieve. What sizing does is partially seal off the wood, lessening stain penetration, which is usually the exact opposite of what we want when staining**.
*And I would recommend using dilute shellac or varnish for this instead of the commercial product because A, you can use the final finish you're going to be applying to the wood anyway, B, you know exactly what you're using and can control its strength to perfectly suit the wood you're using, and C, because it'll almost always be far cheaper.
**Where a lighter staining effect is desired for any given stain colour, simply diluting the stain is often a more direct way of achieving the same result.