From page 240 of the Player's Handbook, find steed says, in part:
You summon a spirit that assumes the form of an unusually intelligent,
strong and loyal steed, creating a long lasting bond with it.
Appearing in an unoccupied space within range, the steed takes on the
form that you choose, such as warhorse, a pony, a camel, an elk, or a
mastiff….When the steed drops to 0 hit points, it disappears, leaving behind no
physical form. You can dismiss your steed at any time as an action,
causing it to disappear. In either case, casting this spell again
summons the same steed, restored to its hit point maximum….
It seems convenient for it to show up with the regular attire of a mount, and to have that stick with it. However, this basically gives it the functionality of the "leomund's secret chest" (p254) which is a level 4 spell with a 5,050gp cost.
What's to stop a player from loading the summoned warhorse with luggage and dismissing it until they feel like accessing it again?
I am not looking for "it's up to the DM" type answers. Rules as intended are best, rules as written will substitute, and info from official WotC sources is great too.
Best Answer
Rules as intended, it's the DM's choice. Jeremy Crawford, lead rules designer of D&D 5e, was asked this exact question, and said that it's up to the DM. Not what you wanted to hear, but you did say rules as intended was the best type of answer. Weirdly, he has since said that the steed leaves gear behind.
RAW, on the other hand, would suggest that your steed can't take anything with it.
"It" disappears, not "it and everything it's carrying". And when you cast the spell again, "the same steed" appears, not "the same steed and everything it was carrying last time it disappeared".
It's also worth considering that "you summon a spirit", which "assumes the form" of a steed. It's not a physical steed that you're summoning and dismissing, so it doesn't make all that much sense for you to be able to summon and dismiss physical objects along with it.