While no DOCSIS revision has symmetric theoretical caps, DOCSIS 3.1 which is currently in use is capable of up to 2Gbps upstream, while the upcoming (for the most part) DOCSIS 4.0 standard is capable of up to 6Gbps upstream, and was developed largely to response to increasing demands for upstream bandwidth. While these are obviously best-case speeds, gigabit symmetric is quite achievable with DOCSIS3.1 and available in some markets.
However, it is unfortunately true that many cable carriers are constrained in their channel planning by legacy devices with fixed channel requirements. For example, Comcast has at least historically struggled with planning DOCSIS channels around legacy Motorola STBs that require their on-screen guide information on a specific channel. Essentially, in order for Comcast to switch to full DOCSIS3.1 upstream channel allocations they will need to first get every customer with a legacy STB to swap it out for a new one. This is a formidable task and not one they have so far completed, so in my area upstream remains limited to 35Mbps even with 1Gbps downstream.
However, it is unfortunately true that many cable carriers are constrained in their channel planning by legacy devices with fixed channel requirements. For example, Comcast has at least historically struggled with planning DOCSIS channels around legacy Motorola STBs that require their on-screen guide information on a specific channel. Essentially, in order for Comcast to switch to full DOCSIS3.1 upstream channel allocations they will need to first get every customer with a legacy STB to swap it out for a new one. This is a formidable task and not one they have so far completed, so in my area upstream remains limited to 35Mbps even with 1Gbps downstream.