This is what I know about the subject.
Compounds in tea, green and black, AND coffee do inhibit iron absorption. but only in the nonheme sources of iron. It doesn't ' affect or by not very much heme iron absorption. This can be a real problem for vegans who drink a lot of coffee or tea. Especially if they are in the habit of drinking tea and coffee at every meal. In one study of a non-vegetarian man who had anemia the anemia WAS the result of tea drinking. he drank 1.5 liters of tea a day. That's like 8 glasses of tea every day.
First off, checking iron levels is pretty easy to do. It's included in almost any annual checkup. I have been giving blood every 6 weeks or so, and they always check my iron levels. And I am almost always borderline anemic. even before I was vegan or vegetarian - this was true. Although since becoming vegan my nutrition awareness is much higher. I also drink less coffee or tea than I used. But of course, I eat a lot less meat than I used to.
If iron is a concern (and that would be because you had low levels in a blood test), and you don't want or can't give blood, then just get a home iron level test. they are simple, cheap, and you only need one drop of blood.
there are some good ideas that anyone can implement. Don't drink coffee or tea at meals. It's said the best time to get your caffeine fix is not first thing in the morning - but mid-morning. Like coffee break time. And that also will keep your coffee or tea from inhibiting your iron absorption at breakfast. Also, for lots of good reasons its good to moderate your tea and coffee consumption.
If you do take an iron supplement don't take it with a cup of coffee or tea. Best bet is to follow it up with a meal or a piece of fruit. OJ does promote iron absorption. but modern day grocery store OJ is awful stuff. If you can have freshly squeezed juice. or just have an orange or any piece of fruit.
Iron is toxic but excess iron is not going to kill you. Your body is very well equipped for eliminating excess iron in your stools. In fact, your stools are really good indicators of getting too much iron (but not too little). When your body eliminates iron through your stools, your stools will turn black and get harder. When your poop starts to resemble iron rods (JK) you need to back off the supplements.
and you probably shouldn't take a supplement unless you have been diagnosed with anemia or just a low blood iron level. but low iron levels are easy to "treat" with better nutrition. if you do take a multi, you can take one without iron. but if you are like me and are always borderline anemic you might find a multi with just a little bit of iron. Multis with 100% of the RDA of iron turn my stools black and rock hard. but I'm ok with multis that have 30 - 50%.
If you are a vegan and you use something like Cronometer to monitor your nutrition, your goal should be over 100% of the mRDA of iron. Not only is nonheme iron less absorbable, but also a lot of the foods we eat interfere with iron absorption.
don't forget that cooking in cast iron does provide you with some iron.