The Pokemon anime is unique from many others as it has simultaneously changed a lot and not at all over its 23 year run. Certain aspects, like Ash's age and his primary partner, Pikachu, have endured throughout everything, but other things like the friends he travels with have changed drastically.
Old school Pokemon fans no doubt fondly remember the days in Kanto when Ash travelled alongside Brock and Misty, but since then his entourage has changed nearly every season. While this is fairly normal, the most shocking change by far was the replacing of Brock with a new character, Tracey.
For the longest time, fans didn't know why this decision was made, as Brock was something of a fan favorite character. Because of this public outcry, Brock was eventually reinstated, but the reason for his brief leave of absence wasn't very clear until a 2008 interview with Masamitsu Hidaka in which the director wen into greater detail about the reason for the shake up.
As it happens, the actual reason for Brock's departure was fear from the localisation team that Brock might be perceived as racist by American audiences. One of Brock's most notable features are his eyes (or lack thereof), a feature which the showrunners were worried could be interpreted as a sort of racist stereotype. In the end it seemed this wasn't the case, as Pokemon added Brock back in after fans voiced their disappointment in the change. Since then he was one of Ash's most consistent travelling partners.
To contextualize this, it's important to look back at the debacle with the Pokemon Jynx. As the franchise was getting its footing in the western market, American audiences were outraged by the appearance of Jynx, who seemed to represent black face due to its dark skin and exaggerated facial features. Coming off the back of this controversy, it's understandable that Pokemon would want to be a bit more careful in the future when it comes to cultural and racial matters.
Though he isn't part of Ash's permanent crew, Brock has made appearances in the Pokemon anime more recently. As far as his replacement, Tracey wasn't so lucky. This new character tagged along during the events of the Orange Islands saga but has since not made another major appearance and is hardly mentioned. Perhaps if he was introduced in another way rather than as Brock's replacement he might have faired better, but that unfortunately was not the case.
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Source: Water Pokemon Master/PokeBeach