Lots of times I don't particularly have anything written down as DM and am winging it ("expletive, I wasn't planning on them going there... what will be there"). Obviously in those cases what I decide to put in is related to their health and things. What makes that materially different than fudging by upping/downing the hitpoints of something in a room that they are getting to? Is that much different in effect than fudging?
To me it is. In both impromptu and planned encounters, you are estimating wat will be a fun encounter. Fudging is changing that part way through the encounter. To me, its not having faith in the players being able to account for their current situation and adapting to it.
So, you plan a CR 12 Deadly encounter yet when the party reaches it they are spent, it is there job to know they can't handle it and to avoid it or retreat or deal with it in another way. No need for you to remove their agency and the importance of their choices (good or bad) by fudging.
In an impromptu encounter you decide a CR 10 moderate encounter is appropriate and not a CR12 Deadly.
In both cases you are making a plan based upon your current information. In both cases you might be right or wrong as to how hard it will be. But in both cases the party is presented with a challenge and gets to deal wit it appropriately.
Should the answers to all of those be written down before the combat starts to avoid having to decide on the spur of the moment when influenced by how it is going? Is not doing so much different in effect than fudging?
No. You are making rulings, not adjusting a plan just to accommodate die rolls.
If the problem is swinginess of dice, is it bad if the DM either has some fixed rule to ameliorate it (will have monsters have possible fumbles but not crits if party is doing bad, and vice versa if good) or have a DM pool of inspiration (one per session? two per session) to balance things out? Is that too fudgey?
Fixed rules are fine. If you don't need a die roll, don't roll one. DM inspiration? Well, imo if it is to help out the party, then that's bad. Because once again you are removing player agency and the importance of their good and bad decisions.
So, question - did the GMs who made you feel this way discuss using these techniques before play began?
It's happened more than once. But no, they did not. Because like many in this thread, they thought fudging to increase fun was part of their toolkit and felt no need to explain it. Just like most DM's would not bother to pre-explain that they might use random encounters or random treasure generation, why would pre-discussing fudging be needed?
(Not that I agree, but that is the viewpoint it usually comes down to.)
I will say when I start a campaign that I do discuss it. That I generally I don't fudge, or insure that they can beat every encounter through combat, or that they are going to "win" every situation. That I plan to lay out situations, and react to their actions with what it seems logical for the factions to do, come what may - within expectations that in general they will be the heroes of the story.