Remember, this is the company that thought a Toughness reduction on an Equipment with a powerful on-death trigger was a downside. It wouldn't surprise me in the slightest that Oko was broken because they literally didn't think of the Elkening as removal. They've also gone on record as having completely lost track of why they balanced a card a certain way, resulting in Siege Rhino, which was supposed to counteract something they wound up axing and they just never removed the abilities in question that no longer had a competing mechanic to balance them out.
There's a very real possibility they'd started with Broko being 3 mana, with the experience of War of the Spark 3-mana 'Walkers being overwhelmingly played and intending Broko to be a universal inclusion in Simic Food, as a result of Blue Artifact + Green Food, and proceeded to end up breaking him with simultaneously increasing his starting Loyalty and increasing his Loyalty generation somewhere along the line.
Perhaps the intended outcome was that you wouldn't be using the Elkening as constant removal because they weren't expecting people to be easily defending from that stream of 3/3s, but lost track of token generation, ramp totals and the overall commonality of bigger creatures and greater threats that negated that bit of design by having the 3/3 Elk be an easily-dealt-with result and less of an issue than what you turned it into from.
Also, I second the on-attack mana needing to be RW, if not RR, and it doesn't work as intended because the mana occurs upon declaring the attack, so the Flash creatures it goes with (because the mana leaves the pool as the declare-attackers step ends) won't be able to do any attacks. The extreme narrowness of when you can spend it and lack of ambush access means it can get away with not being restricted to CMC 2 or less, as you can only use it on Flash creatures, vastly preferably with Haste, which is a very narrow selection. Lastly, the hardcore aggression of it and lack of defensiveness means it isn't a good fit in White in the slightest Combining these issues and fixes for them:
Boros Recruiter 1w
Creature - Human Soldier
At the beginning of each combat phase, add RW. Spend this mana only to cast creature spells.
I'm still uncomfortable with this, though that's because this sort of mana (situational and/or temporary ramp) is actually in Red's color pie as recently as Eldrain with Irencreg Feat, so the continued color pie bend has less justification. I'd prefer the effect be "At the beginning of each combat phase, add R. Spend this mana only to cast creature spells, and creatures cast with this mana cost W less". Discounts seem to be fine in any color, as long as what they're discounting is in-color to cast easier. It's also innately more restrictable than adding mana, as one can easily exclude generic or colored costs as desired by having it be inapplicable discounts, and even discount colorless-specific costs like Eldrazi without affecting generic costs.
However, the usage of highly specific ramp in replacement of limited-use discounts can be more clear and usable, as mana does clear out of pools as steps and phases end, while you can still keep to firmly on-color effects. For an example of this style of color pie bend:
Response Procedure wwuu
Spells cast when the player who cast them would not be able to cast a sorcery cost 1 more. Whenever a spell's cost is increased this way, add U to your mana pool.
In effect, this discounts Blue instants cast in response to a greater number of enemy instants, but as it emulates this effect with adding mana (also breaking the symmetry of the tax if you have something else to spend that mana on), it naturally constrains the overall discount to same-phase, without additional wording.