Hi Wanderlust,
I'm still feeding an (almost) 16 month old boy. I fed completely on demand until I 'day-weaned' him so I could go back to work, when he was 10 months old. This involved slowly dropping all daytime feeds over a period of about 8 weeks. Now I feed when I get home in the evening, during the night (we bed share) and in the morning before I go to work. I suppose what I'm trying to say is that if you wish to drop the amount of feeds per day, you will have to actively work on that.
For a while on the days I wasn't at work and with him all day, I operated under the 'don't offer, don't refuse' way of feeding which meant I just didn't offer the breast at any stage, but if he came and looked for milk, I would give it to him. Sometimes if he's unwell or going through a growth spurt I still do this. This might be an option but you need to be careful you don't get blocked up if your child just doesn't ask for long periods of time.
It's a really personal choice, and if you're still happy to feed as she wishes at 21 months then just keep going. The WHO states the best thing for children is to breastfeed 'to 2 years and beyond' so it is advised and it is normal. But if you want to reduce, without causing distress to your child, and potential problems such as mastitis or some such to yourself, you need to plan and work on dropping one feed a day, say, the 11am feed, and once your child is getting used to not feeding at that time and you feel it's not causing problems, you can solidify that you just don't ever feed at that time any more, and go on to dropping the next feed, say the 3pm one.
Breastfeeding at this age provides so much for toddlers - it's still a valuable source of nutrition, its still excellent for their immune system, for their mental health, their emotional regulation and it actually enhances their cognitive development - so you're really doing so well by your child to still be going.
This turned out to be a lot longer than I meant it to but hopefully there's something you will find useful in there! 